1. Field of the Invention
The present invention relates to an improved abrasive article having a diamond-like carbon coating layer.
2. Discussion of the Related Art
It has been proposed to improve the wet or lubricated grinding ability of a coated abrasive article by forming one or more oxide-forming metal layers or metal oxide layers on the surface of abrasive particles after the particles have been applied to the make coat of a coated abrasive article, such as described in U.S. Pat. No. 3,508,890 (Fontanella). The metal, usually upon being heated in air, converts to metal oxide. A second and third layer of metal oxide can optionally be coated. The metal oxide layers, usually relatively thick at 1500 to 10,000 Angstroms, act as the size coat. The process is preferably done via vacuum deposition. Also, U.S. Pat. No. 3,992,178 (Markoo et al.) describes forming a graphite outer layer on a flexible abrasive article as an electroconductive layer used to dissipate electrostatic charges that may build-up in the abrasive article during service.
It also is generally known to coat a substrate material with polycrystalline diamond films where the diamond precursor material was condensed from a vapor phase as a continuous film onto the surface of substrate. The phase of the carbon is controlled during formation by known methods to produce a desired metastable diamond state having long range crystalline sp.sup.3 orbitals so as to avoid the tendency of the carbon material to otherwise assume the more thermodynamically stable but undesired state, i.e. graphite, having a high fraction of sp.sup.2 carbon orbitals.
Diamond-like carbon films, commonly referred to as "DLC" films, are coatings that can be produced in generally the same manner as polycrystalline diamond coatings and have the same properties, but unlike diamond coatings, diamond-like carbon coatings are amorphous rather than crystalline. The DLC films also have excellent properties usually associated with abrasives, such as low surface friction and high hardness. Naturally occurring crystalline diamond is usually formed from a network of sp.sup.3 carbon orbitals, arranged in a local tetrahedral symmetry yet maintaining long range crystalline order. However, amorphous diamond-like carbon, or "DLC" for short, has a random alternation between tetrahedral sp.sup.3 and hexagonal sp.sup.2 carbon orbitals. In addition, there is no detectable long range order of these orbitals in DLC (T.E.M. measurements have placed an upper limit of 50 .ANG. for long range order).
In general, two known categories of diamond-like, amorphous carbon films can be deposited on an article, depending on the origin of the carbon source and deposition process. Hydrogenated DLC films are obtained from hydrocarbon sources, such as CH.sub.4, which, for instance, are plasma deposited, whereas non-hydrogenated DLC films are obtained from solid carbon, such as graphite. Hydrogenated DLC films are comprised of molecule segments (C-H) with the carbon matrix having about 0.2 to 0.6 atom fraction hydrogen within the matrix. Plasma deposited amorphous hydrogenated carbon typically has a density of about 2.0 g/cc and a hardness of about 10-40 GPa.
On the other hand, non-hydrogenated DLC films have a disordered arrangement of tetrahedral and hexagonal bonds. Non-hydrogenated DLC films are produced essentially from pure carbon with little or no hydrogen, preferably from a highly ionized source where the ion fraction is preferably greater than 50%. The non-hydrogenated diamond-like carbons like their hydrogenated counterparts, do not have any long range order, however they do have a significant fraction of pure carbon sp.sup.3 bonds. The ratio of the sp.sup.3 bonds to Sp.sup.2 bonds determines the material's physical properties.
The sp.sup.3 /sp.sup.2 bond ratio can be estimated from the plasmon energy determined by electron energy loss spectroscopy (EELS). The plasmon energy is proportional to the square root of the atom density. Diamond has a greater atom density and thus plasmon energy than graphite. Polycrystalline graphite has a plasmon energy measured by EELS of about 25 electron volts (eV). Diamond has a plasmon energy of about 33 eV. Non-hydrogenated diamond-like carbon has a plasmon energy between about 26 and 32 eV, the higher plasmon energies corresponding to higher atom densities which are believed to be due to an increased sp.sup.3 bonding component. Also, a non-hydrogenated amorphous diamond-like carbon coating typically has a density of about 3.0 g/cc and a hardness of about 100 GPa, whereas natural diamond typically has a density of about 3.5 g/cc and a hardness of about 100 GPa.
Hydrogen-free non-crystalline diamond-like carbon coatings can also be produced using cathodic arc plasma deposition, a process which provides a highly ionized carbon plasma, high deposition rates and allows control over the incident ion kinetic energy and the substrate temperature. The sp.sup.3 /sp.sup.2 bond ratio is believed to be dependent on the ion fraction and the incident kinetic energy. Cathodic arc plasma deposition can produce diamond-like carbon coatings with higher plasmon energies than alternative processes, thus, yielding higher sp.sup.3 /sp.sup.2 bond ratios.
A cathodic arc discharge occurs when a high current power source is connected between two sufficiently conductive electrodes and the electrodes are momentarily in contact, either physically or by another discharge. Arc spots form on the cathode surface as the electrodes are separated. These small, luminous regions are often very mobile and move rapidly over the cathode surface. Due to the high current density contained in each spot, rapid ebullition of the cathode material occurs, and this plasma material can be confined, transported using magnetic fields and deposited onto substrates. The current density at each spot can reach 10.sup.6 to 10.sup.8 amperes per square centimeter and this is believed to contribute to the ionization of a large amount of the outflowing vapor.
As to more specific known abrasive applications of diamond and DLC film coatings, there have been many attempts to coat diamond films and diamond-like carbon films of various thicknesses onto individual loose abrasive particles and then utilize the particles in abrasive articles.
For instance, published Japanese Application No. JP 63-284285 (21 Nov. 1988) coats silicon carbide grains with a layer of diamond via chemical vapor deposition to increase the wear resistance of the grain. The silicon carbide is preferably 5 to 50 micrometers in size and the coatings are preferably 0.1 to 50 micrometers thick. It is said that a coating less than 0.1 micrometer shows no improvement in wear resistance. Also, published Japanese Application No. JP 1-113485 (2 May 1989), which matured into Japanese Patent Publication No. 6-29401 (20 Apr. 1994), describes alumina, zirconia, or silicon carbide abrasive particles coated with diamond or cubic boron nitride via chemical vapor deposition processes. The particles are described for use in grinding wheels, cutting blades, and finishing work. The coatings are 0.5 to 10 micrometers thick.
However, there are several disadvantages in applying a diamond or diamond-like carbon coating over the surface of abrasive particles on an individual basis. Abrasive particles, especially the smaller particle sizes, have a relatively high surface area. In addition, these coatings are typically applied by physical vapor deposition (PVD) or chemical vapor deposition (CVD) and, as such, these processes are fairly expensive. The large surface area coupled with typical deposition rates, results in a considerable amount of time, and thus cost, needed in order to apply the coating to loose abrasive particles since the coating techniques tend to be directional. In addition, it can be difficult to obtain a uniform coating over the entire surface area of loose abrasive particles.
Also, both diamond films and diamond-like carbon films have been used as a surface coating for nonparticulate articles such as electrical components, wafers, optical lenses, and cutting tools. For instance, U.S. Pat. No. 4,981,717 (Thaler) mentions forming certain diamond-like carbon coatings said to be excellent, when made in larger deposits, as abrasive coatings and being suitable for grinding wheels, among other things. Yet, Thaler only exemplfies coating inorganic materials such as metal and glass as the mentioned substrates. Also, U.S. Pat. No. 4,842,937 (Meyer et al.) coats an abrasive cutting tool surface with a succession of diamond-containing layers to form a wear-protective layer. The multiple layers are applied by chemical vapor or plasma deposition one on top of the other so that the uppermost layer is a diamond film, and each following layer has a different diamond content than the preceding layer. Nickel, cobalt or graphite is described as being usable as binding material in the layers.
Also, U.S. Pat. No. 4,992,082 (Drawl et al.) describes forming a diamond or diamond-like carbon coated tool having several layers of CVD diamond or diamond-like carbon particles separated by interposing and covering planarizing layers described as being nonorganic nonresinous materials such as transition metals, transition metal carbides, boron, boron carbide silicon, silicon nitride, and silicon carbide.
U.S. Pat. No. 4,974,373 (Kawashima et al.) discloses the making of coated abrasive articles having a monolayer of abrasive particles. The abrasive article can be coated with a thin film, e.g., a hard carbon, formed by plasma synthesis or other technique so as to protect the outer tool surface. Plasma deposited amorphous "hydrogenated" carbon (i.e., only 30-60% sp.sup.3 bonded fraction) and techniques for making same were known and available at the time of Kawashima et al. However, plasma deposited amorphous hydrogenated carbon has a relatively low hardness of only about 10-40 GPa.
U.S. Pat. No. 5,401,543 filed 11 Nov. 1993 (O'Neill et al.) and assigned to the assignee of the current invention, teaches a method of making diamond-like coatings and coated articles made thereby, such as semiconductors, with the use of vitreous graphite or glassy carbon as a cathode material for cathodic arc coating.
It is not believed that the art has heretofore reported the provision of a layer of diamond-like carbon as a layer of a coated abrasive article.